左连接不起作用

Question

我写了以下查询

SELECT 
   us.Id as Id, us.Name as Name,
   SUM(CASE WHEN c.isPublish = 0 THEN 1 ELSE 0 END) AS PendingCoupons,
   SUM(CASE WHEN c.isPublish = 1 and convert(date,c.PublishedDate,101) >=  convert(date, GETDATE(), 101) THEN 1 ELSE 0 END) AS ApprovedCouponsToday,
   SUM(CASE WHEN c.isPublish = 0 and convert(date,c. CreateDate, 101) = convert(date, GETDATE(), 101) THEN 1 ELSE 0 END) AS PendingCouponsToday,
   SUM(CASE WHEN c.isPublish = 1 THEN 1 ELSE 0 END) AS ApprovedCoupons,
   SUM(CASE WHEN c.isPublish = 1 and c.Userid = us.Id and convert(date, c.PublishedDate, 101) >= convert(date, GETDATE(), 101) THEN 1 ELSE 0 END) AS ApprovedByUserToday,
   SUM(CASE WHEN c.isPublish = 1 and c.Userid = us.Id THEN 1 ELSE 0 END) AS ApprovedByUser,
   SUM(CASE WHEN c.ReviewVerify = 1 and convert(date, c.PublishedDate, 101) >= convert(date, GETDATE(), 101) THEN 1 ELSE 0 END) AS ProcessToday,
   COUNT(*) AS Total
FROM 
   Users AS us
LEFT JOIN 
   Coupon c ON Userid = us.Id
GROUP BY 
   us.Name , us.Id

我有以下两个表

并且在运行上面的查询后，结果总是这个

此查询中是否有任何错误，因为它总是返回我的数字为“0”而我每个用户都有近100张优惠券，但它没有显示

Answer 1

我认为您可能需要使用RIGHT JOIN从Coupon表中获取所有记录。

Answer 2

似乎自RIGHT JOIN起作用并给出正确的答案（可能是虚假的用户ID），而LEFT JOIN根本没有给出与优惠券相关的任何结果，优惠券是在非现有用户上注册。

LEFT JOIN要求数据存在于表格的左侧，可能存在于右侧，而RIGHT JOIN则相反。换句话说，数据存在于最右边的表格（优惠券）中，与左侧（用户）没有任何连接

Answer 3

您没有使用正确的联接。 您想获得用户ID的优惠券吗？ 使用此联接。 从    用户对我们而言 LEFT JOIN    优惠券c ON us.Id = c.Userid 如果这不起作用，那么使用： 从    用户对我们而言 LEFT OUTER JOIN    优惠券c ON us.Id = c.Userid